This invention relates to systems and methods used to remove acid gas (CO2 and H2S) from a natural gas stream. More specifically, the invention relates to systems and methods designed to remove those acid gases by membrane permeation and not using amines or physical solvents.
The total volume of acid gas removed by traditional acid gas removal technologies such as amines and physical solvents is limited by size of the contactors and the initial installed volume of solvent and the regeneration rates. Because of this, if acid gas removal requirements increase—due to higher acid gas concentrations in the inlet gas—by more than the original design margin (typically between 5% and 10% higher than design basis), more equipment must be added to process the gas. In practice, it is common that for natural gas fields containing concentrations of CO2, it is likely that the CO2 content in the inlet gas will increase over the life of the project. The same is true of H2S.
Gas reservoirs have finite useful production lifetimes. Therefore large users of natural gas, such as LNG plants, electric power generation plants and gas pipelines typically receive produced gas from one or more wells or fields with each well or field having different combinations of natural gas and acid gases over a 20-30 year timeframe. Often gas is supplied from different gas fields with different acid gas percentages and the different gas streams are blended together to form the feed gas.
Because of this, gas compositions change with varying production volumes and acid gas compositions from the individual wells contributing gas to the blended feed gas. This change is amplified by the common practice of gas production companies to produce the most economical, low acid gas wells first, in order to maximize early financial returns. The practice results in producing gas with higher and higher acid gas compositions over time.
The actual long-term production profile of the gas field and acid gas composition profile is often highly uncertain at the time that the initial acid gas processing equipment is installed upstream of the LNG, power plant of the pipeline. As a result, it is common that additional acid gas removal must be added at some later date. In efforts to keep the total cost of the acid gas system low, the acid gas removal systems are often not initially designed to handle inlet gas at the high range of inlet acid gas compositions over the field production lifetime. Therefore, new acid gas processing equipment and associated compression are typically added in phases (e.g., Phase 1, Phase 2, . . . Phase N), which adds considerable capital cost to operations.
A need exists to keep the size of the initial acid gas processing system as small as practical, but still flexible enough to avoid adding additional equipment to that system over time as acid gas concentrations increase in the feed stream.